1. Field of the Invention
This invention relates to a magnetic recording medium and more particularly, it is concerned with an improved magnetic recording medium having an S/N ratio increased in high density recording.
2. Description of the Prior Art
Up to the present time, various trials have been made to increase the recording density of a magnetic tape (relative speed of tape and head/maximum frequency of recording signal) from the requirements as to the high fidelity in audio cassettes and the low speed in small-sized video tape recorders. In these trials, for example, it is proposed to increase the coercive force of a magnetic layer, to increase simultaneously the residual magnetization and coercive force as in metallic cassettes or to arrange a magnetic layer with a high coercive force as an upper layer to increase the output at a high recording density with preventing the output at a low recording density from lowering.
Lately the thought of vertical magnetization recording has been introduced to use effectively the residual magnetization component in the vertical direction to the surface of a magnetic recording medium. It is said that according to this vertical magnetization recording, the recording density as defined above becomes high, the output decrease due to self-demagnetization (decrease of the residual magnetization by a self-demagnetization field acting in a direction to negative the self-magnetization between N and S poles formed by the self residual magnetization, which is increased with the decrease of the distance between the N and S poles) is not so large if the recording wavelength is smaller than the thickness of a magnetic layer, and the high density recording is effective with a recording wavelength of 1-2 microns or less. As a medium for the vertical magnetization recording, there have been used at present materials capable of being readily magnetized in the vertical direction, such as sputtered film of Co-Cr alloy, and two layer structures having a magnetizable layer of a permalloy with a coercive force under this layer, which have, however, disadvantages in material and operation.
In magnetic recording media with magnetic layers of coating type, utilization of a slant or vertical magnetization component, not in parallel with the magnetic surface, has been proposed as described in, for example, U.S. Pat. Nos. 3,185,775 and 3,052,567 and Japanese Patent Publication No. 15203/1974. However, these methods have the drawbacks that the noise level is still high and the output is low.
A magnetic tape having a two layer structure using acicular magnetic particles with a large length/width ratio has hitherto been known wherein in the lower layer, the particles are oriented in parallel with the surface of the support and in the upper layer, they are oriented vertically to the surface to thus raise the short wavelength sensitivity. In practice, however, it is difficult to stand the acicular particles vertically in a step of coating and drying the particles and rather, the acicular particles tend to fall down to be in parallel with the surface with the decrease of the thickness due to vaporization of a solvent during drying. This tendency is increased with the degree of anisotropy, i.e. with the increase of the length/width ratio.